1. Field of the Invention
This invention relates to mold components and a mold assembly, for molding a substrate for an optical recording medium.
2. Description of the Related Art
Conventionally, as a mold assembly for molding a substrate D3 (see FIG. 15) for an optical recording medium, such as a CD (Compact Disc) and a DVD (Digital Versatile Disc), a mold assembly has been known which includes a fixed mold having a stamper set thereon for forming micro asperities, such as grooves, in a surface of the substrate D3, and a movable mold having a sleeve set therein for forming a central hole D3h (central hole for mounting the optical recording medium) in the substrate D3. As shown in FIG. 15, the substrate D3 molded using the mold assembly has the central hole D3h formed in a central portion thereof, and micro asperities, such as grooves, not shown, formed in a predetermined area on one surface (upper surface, as viewed in FIG. 15) thereof.
In the above case, the optical recording medium, not shown, is completed by sequentially forming various functional layers (reflection layer, recording layer, protective layer, and so forth) in the predetermined area on the one surface of the substrate D3. In doing this, part (protective layer, for example) of the functional layers is formed by coating resin by a spin coating method, and curing the coated resin by a predetermined curing treatment. Now, it is known that to radially coat the resin substantially uniformly by the spin coating method, it is preferable to drop the resin in the vicinity of the center of the substrate D3 (toward the center within the central hole D3h). To this end, as shown in FIG. 15, a method is proposed in which resin is coated using a disc-shaped member 72 as a resin-coating assisting member. According to this method, the disc-shaped member 72 is placed on the substrate D3 such that it covers the central hole D3h, and the substrate D3 is spinned while the resin is being dropped from a nozzle N onto the disc-shaped member 72, whereby the resin R is coated on the substrate D3. In this method, however, maintenance of the used disc-shaped member 72 (e.g. cleaning of the disc-shaped member 72 having the resin R stuck thereto) is troublesome, and hence the present inventors have already developed a mold assembly 51 (see FIG. 11) for molding a substrate D4 (see FIG. 14) which is formed with a small-diameter central hole D4h and a hollow cylindrical protruding portion D4r formed in a central portion thereof, thereby enabling the resin R to be dropped in the vicinity of the center of the substrate without using a resin-coating assisting member, such as the disc-shaped member 72.
Referring to FIG. 11, the mold assembly 51 includes a fixed mold 61, and a movable mold 21 moving to and away from the fixed mold 61. The mold assembly 51 is configured such that a substrate D4 shown in FIG. 14 can be molded by injecting a molten resin material into a cavity Ca2 defined between the molds 61, 21 which are closed, from an injection molding apparatus, not shown. As shown in FIG. 11, the fixed mold 61 is comprised of a fixed-side mounting plate 12, a fixed-side mirror 63, a sprue bushing 14, a stamper holder 65, and a stamper 66. The fixed-side mounting plate 12 is configured such that it can be mounted on a fixed-side platen of the injection molding apparatus, and has a hole 12a circular in cross section formed through a central portion thereof. The fixed-side mirror 63 is configured to have a disc shape and mounted to the fixed-side mounting plate 12, with a hole 63a circular in cross section formed through a central portion thereof. In this case, as shown in FIG. 13, a plurality of vertical grooves 63c are formed in an inner peripheral surface of the fixed-side mirror 63 which defines the hole 63a. Further, as shown in FIG. 13, the fixed-side mirror 63 has holes 63d formed therethrough such that the holes 63d communicate with the vertical grooves 63c, respectively. The holes 63d serve as air-sucking passages for sucking air to attract the stamper 66 to a mirror surface 63b (surface of the fixed-side mirror 63 toward the cavity Ca2), and communicate with piping, not shown, of a vacuum pump outside the mold assembly 51.
As shown in FIG. 11, the sprue bushing 14 has a hole 14a formed through a central portion thereof, and is fitted into the holes 12a, 63a. The stamper holder 65 is configured to have a hollow cylindrical shape and mounted between the sprue bushing 14 and the fixed-side mirror 63. In the mold assembly 51, as shown in FIG. 13, small gaps are formed between the inner peripheral surface of the stamper holder 65 and an outer peripheral surface of the sprue bushing 14, and between an end face 65b of the stamper holder 65, opposite to an end face thereof facing toward the cavity Ca2, and a surface 14c of the sprue bushing 14. When the movable mold 21 is separated from the fixed mold 61 during molding operation, the gaps function as air-blowing passages for blowing air to release the molded substrate D4 from the fixed mold 61. The gaps communicate with piping, not shown, of a blower outside the mold assembly 51 via a gap between the surface 14c of the sprue bushing 14 and the fixed-side mirror 63, and a gap between the fixed-side mounting plate 12 and the fixed-side mirror 63. Further, as shown in FIG. 13, along the entire periphery of the stamper holder 65 toward the cavity Ca2, there is formed a ring-shaped holding piece 65a for holding the stamper 66 in a state pressed against the fixed-side mirror 63 (this construction is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 9-193206). In this case, as shown in FIG. 13, the holding piece 65a protrudes toward the cavity Ca2 when the stamper holder 65 is mounted.
Referring to FIG. 12, the stamper 66 is configured to have a disc-like shape, and an insertion hole 66h having a diameter slightly larger than an outer diameter of the stamper holder 65 is formed through a central portion of the stamper 66. Further, the stamper 66 has a groove-forming surface 66b (lower surface as viewed in FIG. 12) toward the cavity Ca2 configured such that micro asperities, such as grooves, can be formed in a surface of the substrate D4 during molding. The stamper 66 is positioned by the stamper holder 65 such that it is fixed to a predetermined location on the mirror surface 63b (surface toward the cavity Ca2) of the fixed-side mirror 63, and when air is sucked, the stamper 66 is attracted and fixed such that the inner peripheral portion and a mounting surface (upper surface as viewed in FIG. 11) thereof are brought into intimate contact with the fixed-side mirror 63. In mounting the stamper 66 on the fixed mold 61, the stamper holder 65 is inserted into the insertion hole 66h of the stamper 66, as shown in FIG. 12. In this case, since the insertion hole 66h is formed to have a diameter slightly larger than the outer diameter of the stamper holder 65, the stamper holder 65 is smoothly inserted into the insertion hole 66h. Then, while holding the stamper 66 using the holding piece 65a, the stamper holder 65 is fitted between the sprue bushing 14 and the fixed-side mirror 63. In this state, as shown in FIG. 13, since the holding piece 65a covers the gap between the inner peripheral surface of the stamper 66, defining the insertion hole 66h, and the outer peripheral surface of the stamper holder 65, burrs due to the gap are prevented from being formed on the substrate D4. By fixing the stamper 66 by the above method, it is possible to attach the stamper 66 to the fixed mold 61 without directly touching the groove-forming surface 66b thereof. This prevents the groove-forming surface 66b from being scratched or flawed.
The movable mold 21 is comprised of a movable-side mounting plate 22, a movable-side mirror 23, an outer peripheral ring 24, an ejector sleeve 25, a gate cutting sleeve 26, and an ejector pin 27. The movable-side mounting plate 22 is configured such that it can be mounted to a movable-side platen of the injection molding apparatus. The movable-side mirror 23 is formed to have a disc-like shape, and mounted to the movable-side mounting plate 22. The outer peripheral ring 24 is formed to have a hollow cylindrical shape and fitted on the outer periphery of the movable-side mirror 23. The ejector sleeve 25 is formed to have a hollow cylindrical shape and slidably mounted in holes formed through respective central portions of the movable-side mounting plate 22 and the movable-side mirror 23. The gate cutting sleeve 26 is formed to have a generally hollow cylindrical shape and slidably fitted in the sleeve 25. The ejector pin 27 is formed to have a columnar shape and slidably fitted in the gate cutting sleeve 26.
When the substrate D4 is molded using the mold assembly 51, first, the fixed mold 61 is mounted to the fixed-side platen of the injection molding apparatus, not shown, and the movable mold 21 is mounted to the movable-side platen of the injection molding apparatus. Then, driving means of the injection molding apparatus is operated to close the fixed mold 61 and the movable mold 21, as shown in FIG. 11. Then, a molten resin material is injected into the cavity Ca2 through the hole 14a of the sprue bushing 14. After the resin material in the cavity Ca2 is sufficiently cooled to be solidified, the driving means of the injection molding apparatus is operated to separate the movable mold 21 from the fixed mold 61. At this time, air is blown through the gap between the outer peripheral surface of the sprue bushing 14 and the inner peripheral surface of the stamper holder 65 to thereby release the substrate D4 from the fixed mold 61. Then, the ejector pin 27 and the ejector sleeve 25 are moved toward the fixed mold 61, causing the substrate D4 to be ejected. Thus, the substrate D4 is produced. In this case, as shown in FIG. 14, the substrate D4 has a central hole D4h formed through a central portion thereof. Further, a hollow cylindrical protruding portion D4r is formed around the central hole D4h on one surface (upper surface as viewed in FIG. 14) of the substrate D4. In a predetermined area on the one surface of the substrate D4, micro asperities, such as grooves, are formed by the stamper 66. Further, the substrate D4 has a annular groove D4d formed in a central portion of the one surface of the substrate D4, for which the holding piece 65a of the stamper holder 65 in contact with the resin is responsible.
Next, when the resin R is coated on the one surface (surface on which micro asperities, such as grooves, are formed) of the substrate D4 by the spin coating method to form a functional layer (protective layer, for example), the resin R is dropped from the nozzle N to a point in the vicinity of the outer peripheral surface of the hollow cylindrical protruding portion D4r, as shown in FIG. 14. Then, the substrate D4 is spinned, whereby the resin R is stretched (drawn) up to the periphery of the substrate D4. Subsequently, the coated resin is cured by a predetermined curing treatment. Thus, the functional layer is formed on the one surface of the substrate D4 without using the resin-coating assisting member.
However, after investigation of the above mold assembly 51, the present inventors have discovered the following points to be improved: In the mold assembly 51, since the holding piece 65a formed on the stamper holder 65 protrudes toward the cavity Ca2, the annular groove D4d is formed in the central portion of the one surface of the molded substrate D4. Therefore, when the resin R is coated on the one surface of the molded substrate D4 by the spin coating method, the annular groove D4d impedes the smooth drawing of the resin R, which can cause defects, such as uneven thickness of the resin R and formation of coating streaks. It is desirable to eliminate this inconvenience. To this end, there is contemplated a method that uses a stamper formed to have a reduced outer diameter so as to cause the annular groove to be formed at a location closer to the center of the substrate D4, to thereby prevent occurrence of uneven thickness of the resin R and formation of coating streaks. However, since there is a limit to reduction of the outer diameter of the stamper, it is difficult to completely prevent the occurrence of the above inconvenience by the above method. Further, to dispense with the holding piece 65a, a method is contemplated that causes the stamper 66 to be directly fitted in the sprue bushing without using the stamper holder, to thereby prevent the annular groove from being formed. In this method, however, to enhance operability in mounting the stamper, it is necessary to form the insertion hole 66h of the stamper 66 such that it has a diameter slightly larger than the outer diameter of the sprue bushing 14. If the insertion hole 66h is increased in diameter, since a gap is produced between the inner peripheral surface of the stamper, defining the insertion hole 66h, and the outer peripheral surface of the sprue bushing 14, and at the same time there is no holding piece 65a, burrs responsible for the gap are formed at the central portion of the substrate D4. Further, in this method, in mounting the stamper 66, it is necessary to directly touch the stamper 66, which can cause a scratch or a flaw to be formed on the groove-forming surface 66b of the stamper 66. This presents a hurdle in employing the method.